Abstract

Numerous papers associate the rate of leaf litter decomposition with the presence/abundance of gammarids. However, recent studies showed that Gammarus sp. may have high diet diversity (as animals, moss, algae). In this context, we quantify the diet of Gammarus fossarum to highlight and statistically consider what many authors affirmed rather by observation. To this end, we used a recently described multivariate analysis, called %PCA, which allows investigating data dealing with diet composition. Body size and microhabitat type were revealed highly significant variables governing several parameters of the diet. In particular, our results show that food item size and predatory behaviour increased with body size, whilst the type and abundance of food consumed was related to microhabitat type. Furthermore, body-size had a significant effect on specialisation whatever the microhabitat type. Niche overlap also decreased as pairwise differences in body-size increased, allowing the aggregation of individuals from different instars. We conclude that G. fossarum exhibit an opportunistic behaviour, which makes necessary to reconsider the importance of gammarid "shredding" activity to accurately assess stream functioning and flux/redistribution of energy and matter (leaf litter breakdown). We discuss the functional feeding group membership of G. fossarum at the individual and population level.